Recent Developments in Edible Coatings for Fresh Fruits and Vegetables

Edible films serve as a conveyance for delivering active components, flavors, drugs, and nutraceuticals (Wongphan & Harnkarnsujarit 2020). These active components are trapped in the biopolymer matrix and remain stable until reaching the consumer’s table. Edible films are fabricated from biopolymers using wet and dry processes. The wet process is solvent casting; whereas, the dry process is also known as the extrusion process (Siemann 2005; Suhag et al. 2020). Edible coatings are generally prepared from the substance pertaining to film-forming properties. The raw material for formulating the edible coating must be able to be dispersed and dissolved in the desired solvent, for example water, alcohol and a mixture of solvents in the desired ratio. While preparing edible coating plasticizers, antimicrobial agents, flavor-enhancing substances, vitamins, colors, and spices can also be incorporated (Dhall 2013; Raghav et al. 2016). Plasticizers are blended into the solution of edible coating to increase the mechanical strength (Sothornvit & Krochta 2005) and to provide flexibility to the films and coatings (Hassan et al. 2018). Water acts as a natural plasticizer for edible coatings and films. Glycerol, sorbitol, fatty acids, sucrose, polyethylene glycol (PEG), propylene glycol (PG), and monoglycerides are good options to provide flexibility and plasticizers (Krochta 2002; Sothornvit & Krochta 2005).

The casting method or solvent casting method is commonly exercised at the pilot scale. The main steps involved in this method are solubilization of biopolymer into suitable solvent followed by casting into suitable molds and drying (Jensen et al. 2015; Suhag et al. 2020). The casting method provides fewer defects due to its homogenous nature, and the manufacturing method is cost-efficient (Chen et al. 2008; Yang et al. 2011). The method contributes to appreciable optical purity and good transparency (Suhag et al. 2020). The salient disadvantage of casting methods is that proteins and other components get degraded by the use of solvent, the amount of film fabricated is limited, film development is dependent on the temperature and evaporative range, and it is a time-consuming technique. Therefore, it is not recommended for commercial use (Sait & Ma 2009; Fakhouri et al. 2013; Jensen et al. 2015; Suhag et al. 2020).

The extrusion method (dry process) for the development of edible films is used at a commercial scale. The extrusion process takes place in three different stages: a) feeding stage, b) kneading stage, and c) heating stage. The extrusion process is also referred to as the dry process because the least amount of water or solvent is required during the feeding zone to obtain an optimal result (Peressini et al. 2003; Calderón-Castro et al. 2018). The extrusion method takes a short processing time, least solvent, and low energy expenditure. This method is recommended for commercial purposes, and the final product has high mechanical and optical properties (Raghav et al. 2016). The major drawbacks of using the extrusion method are: its high initial cost, requirements to skilled workers, biopolymer used must be temperature tolerant, and low moisture content (Raghav et al. 2016; Suhag et al. 2020).

The deposition of the edible coating on the food product depends on the nature and the surface attributes of the food product to be coated (Suhag et al. 2020). Edible coatings can be deposited over the fruits and vegetables by various techniques, such as dipping, spraying, fluidized bed, and panning (Raghav et al. 2016; Suhag et al. 2020). The application of an edible coating by the brushing method has been found to have valuable results on both less perishable vegetables such as beans and extremely perishable fruits such as strawberries and berries (Valverde et al. 2005; Raghav et al. 2016). The dipping process involves the immersion of food products directly into the edible coating solution followed by drying in which the solvent gets evaporated (Andrade et al. 2012; Senturk Parreidt et al. 2018). The spraying technique is the widely used technique for the application of edible coating over food products. Droplets are formed and then distributed over the food surface via nozzles. Air spray atomization, air-assisted airless atomization, and pressure atomization are the major spraying techniques practiced at the industrial scale (Andrade et al. 2012; Valdés et al. 2017). The low density or small size dry particles are usually thinly coated using the fluidized-bed technique. This process is categorized into three types; (i) top spray, (ii) bottom spray, and (iii) rotary fluidized bed. The top-spraying fluidized technique is highly effective in food industries (Suhag et al. 2020). The coating or layering solution is sprinkled onto the rotating bowl in which the product is placed. It is followed by drying of the coating layer (Pandey et al. 2006). Hard panning is the application of a hard shell by continuous treatment with sugar syrup on the surface of the product. Soft panning involves the administration of a mixture of corn syrup and sugar for coating followed by the application of dry sugar. Chocolate panning involves the use of fat-based layers such as cocoa-based or white chocolate (Gesford 2002).

Polysaccharides are widely used for the fabrication of edible coatings and films to enhance the shelf life and quality retention of food products. They are proven to possess good oxygen barrier properties but are hydrophilic in nature; edible coatings derived from polysaccharides lack satisfactory moisture barrier properties (Hassan et al. 2018; Yousuf et al. 2018). Polysaccharide-based edible coatings are colorless and constitute less caloric content and in addition to that can be used to extend the shelf life of food products such as fruits, vegetables, and meat (Hassan et al. 2018). Edible coatings resulting from polysaccharides include cellulose and its derivatives, such as methylcellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, methyl ethyl cellulose, carboxyl methylcellulose, starches (derived from different sources), dextrin, pectin derivatives, pullulan, alginate, chitin, and chitosan-based edible coatings, gums – Arabic gum, guar gum, xanthan gum, carrageenan, and agar (Raghav et al. 2016; Hassan et al. 2018; Yousuf et al. 2018; Salehi 2020). The major advantage of using starch-based coatings is that they are colorless, provide an oil-free exterior, it is readily available, and the cost is low, which makes it a promising polymer for an edible coating (Luchese et al. 2017); thus, it can be used to enhance the shelf life of fruits and vegetables. Starch cannot be used solely for the preparation of edible coating due to its hydrophilic nature, which makes it water-sensitive and decreases water vapor barrier capacity; therefore, plasticizers and emulsifiers are added to improve the flexibility and barrier properties (Cazón et al. 2017). Table 1 summarizes the application of various types of edible coatings on fruits and vegetables.

Globular and fibrous proteins happen to be found naturally. Milk protein, that is, casein protein and whey protein, are suitable raw materials for the formulation of edible coatings (Mohamed et al. 2020). Edible coatings and films derived from protein exhibit outstanding barriers against carbon dioxide and oxygen but do not possess great mechanical strength (Yousuf et al. 2018). Almond and walnut oil can be incorporated to enhance the barrier properties (Hassan et al. 2018). Proteins such as soy protein, corn zein, wheat gluten, gelatin are also promising substitutes for protein-based edible films to preserve the quality of products (Hassan et al. 2018; Yousuf et al. 2018). Zein is majorly extracted from corn; it is a raw material for the preparation of edible films and coatings. Protein-based edible coatings derived from zein display significant barriers against moisture, and by the addition of fatty acids, zein coatings can provide a good barrier against water vapor loss (Hassan et al. 2018). Fish protein such as Whitemouth croaker protein isolate along with organoclay Montmorillonite (MMT) demonstrated great potential in extending the shelf life of minimally processed papaya (Cortez-Vega et al. 2014).

Lipids are ideal choices as constituents in edible films and coatings due to the necessity to prevent moisture losses by packaged or nonpackaged food items. Lipids are generally combined with polysaccharides or proteins to improve their functionality in edible films or coatings. The most commonly used lipid compounds in lipid-based edible coatings are neutral lipids, fatty acids, waxes, and resins (Hall 2012; Raghav et al. 2016; Hassan et al. 2018). Coatings derived from paraffin are used to preserve cheese, vegetables, and raw food (Hassan et al. 2018). Fruits and vegetables meant to be stored for a longer time are coated with formulations containing lipids incorporated with antifungal compounds and growth regulators (Hall 2012). Zambrano-Zaragoza et al. (2020) studied the effect of nano-edible coating based on beeswax lipid nanoparticles on strawberries for extending its quality by 21 days of storage at 4 °C. Best results were obtained with 10 g·L⁻¹ of beeswax solid nanoparticles dispersion giving the lowest weight loss (6.1%), a decay index of 31%, and loss of firmness of 34%, being an excellent alternative to increase the shelf life of strawberries. Beeswax or stearic–palmitic acids (1 : 1) mixture was blended with gellan gum to form gellan/lipid composite films. The addition of lipids significantly improved the water vapor permeability but lowered the mechanical properties of the films. Beeswax was found to be a better alternative than the stearic-palmitic acids blend (Yang & Paulson 2000).

Nowadays Aloe vera-based edible coating is emerging in the market demonstrating better results as compared to the traditional edible coatings and health benefits because it also acts as a medicine (Sharma et al. 2019). Aloe vera is reported to have 75 nutrients, sugars, anthraquinones, vita-mins, minerals, salicylic acid, approximately 200 active compounds and amino acids that show medical properties (Dureja et al. 2005). For this reason, a gel-based on this plant was developed that can be used in the preservation of freshness of fruit freshness (Tripathi & Dubey 2004; Misir et al. 2014).

This gel proves to be a harmless and safe alternative to sulfur dioxide, a synthetic preservative that is widely used all over the globe. The Aloe vera gel provides a protective layer against oxygen and moisture transmission from fresh fruits or vegetables and exhibits antibacterial, antiparasitic, antiviral (Misir et al. 2014), and antifungal properties due to which it is capable of inhibiting the microbial deterioration of food. Anthraquinones present in Aloe vera inhibit the solute transport in membranes of Staphylococcus aureus and Escherichia coli strains (Hamman 2008; Lone et al. 2009). It was also reported that Aloe vera inhibits the activity of food-borne pathogenic microbes, that is, Bacillus cereus, Salmonella typhimurium, Escherichia coli, Klebsiella pneumonia, etc. (Misir et al. 2014).

Emodin of Aloe vera is reported to be effective in inhibiting the activity of gram-positive bacteria (Cock 2008). The gel-based coatings have been proven to counteract moisture loss and retain firmness also, facilitating in maintaining the respiratory rate. It also controls fruit ripening, the maturation rate of vegetables, delay oxidative browning, and microbial growth (Castillo et al. 2010). Aloe-vera-gel-coated fruits do not depict a distinct taste and flavor and possess no harm to consumers (Misir et al. 2014). This gel blended with gaur gum for the preservation of Indian jujube (Ziziphus mauritiana Lamk) delayed the ripening process, a significant reduction in physiological weight loss, and visual quality retained for a longer period of time, hence, extending shelf life (Mani et al. 2018).

The formulation of edible coatings using fruit purees was reported first time in 1996 (McHugh et al. 1996). Fruit purees can be intermixed with biopolymers and active compounds to enhance the shelf life of perishable foods (Galus et al. 2020). One of the studies concluded that the shelf life of minimally processed fresh produces, such as pumpkin, carrot, persimmon, etc., can be extended using papaya puree as an edible coating. The papaya-puree-based edible coating can be added into minimally processed papaya to increase its nutritional values (Rangel-Marrón et al. 2019). The application of edible coatings formulated by blending alginate and acerola puree on acerola fruits showed a decrease in weight loss, ascorbic acid, ripening rate, and the decaying process, hence, extending the shelf life of acerola fruits (Azeredo et al. 2012). Papaya purees can be attained from fruit-processing wastes (Otoni et al. 2014). The composite coating of mango puree, gaur gum, sesame protein, and calcium chloride has been proved to enhance the shelf life of fresh-cut mango. The application of the mentioned coating on fresh-cut mangoes showed a decline in degradation of ascorbic acid, total phenolics content, and carotenoids level in the sample and was helpful in maintaining the firmness, total soluble solids, and sensory attributes of mango (Sharma et al. 2019).

Composite edible coatings are a combination of protein/polysaccharide and lipid providing combined advantages. The water-soluble polymers (hydrocolloids) and lipid-based composite films provide the great water vapor barrier property of lipids and good oxygen and carbon dioxide barrier properties of carbohydrates. Bioactive compounds can be incorporated into composite films and coatings to improve their functionality (Sharma et al. 2019). Polysaccharides and proteins exhibit great mechanical and structural properties but lack good moisture barrier properties that can be overcome by blending lipids with them. The main reason behind the applicability of composite films and coatings is to aggregate the advantages provided by carbohydrates, proteins, and lipids into a single resultant coating or film; in addition to that, aggregating also helps in minimizing the loss of each component (Yousuf et al. 2018).

Nanotechnology aids in the postharvest preservation of fresh produce. The nanosized zinc oxide, silicon, and calcium carbonate were used to enhance the shelf life of fresh produce and reduce the decay rate (Jianglian & Shaoying 2013). The exercise of nanotechnology approaches such as the delivery of active or antimicrobial compounds using encapsulation methods has the potential to confront the issue of microbial-caused decay of fruits and vegetables; hence, it aids in maintaining the food quality attributes (Dhital et al. 2018; Jafarzadeh et al. 2021). Emulsification is a process to deliver advantageous compounds to a liquid solution. This approach is employed to make nano-emulsified edible coatings to convey active compounds onto the fresh produce via edible coatings. The application of citral-based nano-emulsified edible coating onto fresh-cut melons has proved to exhibit antimicrobial properties and extended the shelf life of the product (Arnon-Rips et al. 2019).

The implementation of the nanoencapsulation approach by incorporating curcumin and limonene into liposomes has proved to prolong the shelf life of strawberries, whereas the application of limonene liposomes has been found to have significant control over fungal decay and maintain total phenolic content (Dhital et al. 2017). The application of edible coating enriched with limonene liposomes on strawberries demonstrated positive results by extending the shelf life and reducing the respiration rate of food products (Dhital et al. 2018). The employment of solid–lipid nanoparticles with xanthan gum edible coating to preserve tomatoes exhibited positive results to the overall quality parameter of tomatoes (Miranda-Linares et al. 2018). The use of chitosan incorporated with a nanosilicon dioxide coating has proved to be an effective alternative to preserve the food quality attributes. The chitosan with nanosilicon dioxide onto jujube fruit has shown a positive impact in aspects of quality and shelf life (Yu et al. 2012).

The concentration of phenolic compounds preventing microbial-caused decay is high in spices and herbs (thyme, rosemary, oregano, sage). Essential oils (EOs) are majorly recovered from herbs and spices that have high antioxidant properties; therefore, EOs can be used as natural antioxidants as well as provide antimicrobial protection. Active compounds can be extracted from byproducts of plants – pomegranate peel, olive leaves, etc. (Ganiari et al. 2017). Essential oils extracted from spices are commonly used as natural preservatives (Ju et al. 2019).

In the food industry, EOs are commonly used to provide flavor. Along with it, they obtain antioxidant and antimicrobial functional properties. EOs from bergamot, cinnamon, coriander, clove, eucalyptus, lemon, oregano, rosemary, sage, thyme, and tea tree are commonly used to develop edible coatings/films and provide promising antioxidant, antibacterial, and antifungal properties (Sánchez-González et al. 2011a, b). Carvacrol, thymol, and eugenol are the main components present in basil and thyme oils, which exhibit antioxidant properties (Lee et al. 2005). The EOs of oregano (Origanum vulgare L.) exhibit antibacterial properties as it deteriorates Staphylococcus aureus by altering the morphology of its cell surface, which results in loss of cytoplasmic material (de Souza et al. 2010). Oregano EOs repress the synthesis of staphylococcal enterotoxins (de Souza et al. 2010). The EOs also affect the mitochondrial membrane of bacteria (Rasooli et al. 2006). Terpenoid and phenolic compounds are the major components that exhibit antioxidant properties in essential oils. They are extensively used to preserve food (Sánchez-González et al. 2011a, b).

The EOs of tee tree were used as natural anti-oxidants in food, for example in preserving raspberries (Chanjirakul et al. 2006). The EOs of clove, lemon, eucalyptus, tea tree, roomer, and melissa were used in leafy vegetable produces (Ponce et al. 2004) and as factors preserving color, texture, and firmness, in fruits due to reduced water loss and lower respiration rate (Sánchez-González et al. 2011a). Commercially available oranges coated with Lippia scaberrima oil resulted in a reduction in weight loss (du Plooy et al. 2009). The table grapes covered with an edible coating of HPC/chitosan enriched with bergamot oil resulted in a reduction in weight loss (Sánchez-González et al. 2011a). This coating reduced respiration rate in grapes (Sánchez-González et al. 2011a), apples, and melons (Raybaudi-Massilia et al. 2008). Various studies of these authors have shown that the incorporation of EOs into edible coating causes less consumption of oxygen and reduction in carbon dioxide production; the main reason behind this is the lipophilic nature of EO.

Edible coatings are used as the vehicle for the incorporation of bioactive compounds such as antioxidants, probiotics, antimicrobials, and EOs for the improvement of food quality and the creation of functional foods. Bioactive components are present in food in minuscule quantities, and incorporation of these components into a product increases its nutritional value (Tajkarimi et al. 2010; Quirós-Sauceda et al. 2014). Incorporation of N-acetylcysteine and glutathione into edible coatings was found to be effective in preventing browning reactions in fresh-cut pears for a period of 2 weeks without any textural damage (Song et al. 2011). No significant reduction in the number of viable organisms was observed during storage at 5 °C of strawberries coated with encapsulated Lactobacillus acidophilus and Bifidobacterium lactis in calcium alginate and 2% (w/v) solution. Some researchers have used edible coatings to add flavors to food products. Laohakunjit and Kerdchoechuen (2007) incorporated 25% natural pandan leaf extract into 30% sorbitol-plasticized rice starch for coating of nonaromatic milled rice, resulting in produce with a smell similar to aromatic rice. Another approach is by adding flavor precursors in the coating formulations, which, when coming in contact with the food, may react with food components and produce flavoring compounds. An increase in the concentration of aroma compounds was observed when alginate calcium coatings using linoleic acid and isoleucine was applied on apple wedges (Olivas et al. 2012).

Even though edible coatings have been regarded as means of extending the shelf life of fruits and vegetables, still their commercialization faces many challenges. The effect of these coatings on sensory properties sometimes affects consumer acceptance. The incorporation of different herbs, spices, antimicrobials, and antioxidants could impart undesirable flavors to the commodities. Criteria for the selection of edible coating are based on respiration and transpiration rates of food and the storage conditions. Since fruits and vegetables differ in respiration rate, gas diffusion, and skin resistance, coating developed for one commodity may not be suitable for another. The thickness of the coating and method of application affect the permeability of pores, thus influencing transport-related properties. Refined mineral oil-based coatings result in anaerobic conditions, causing fruit injury (Moalemiyan et al. 2012). The anaerobic respiration results in fermentation and development of off-flavor detrimental to the sensory quality of the commodity. The application of essential oils in higher concentrations as antimicrobials in edible coatings may adversely affect the aroma of fresh-cut fruits. Azarakhsh et al. (2012) reported that the use of lemongrass oil in alginate-based coating for fresh-cut pineapple decreased the sensory score even when used at a low concentration. Antibrowning agents such as N-acetylcysteine and glutathione, when used in higher concentrations, may result in unpleasant odors (İyidoǧan & Bayındırlı 2004). Therefore, minimizing the detrimental changes in sensory attributes can be a potential area of research that can be explored.

The quality of living standard is increasing along with the significant demand for fruits and vegetables (Maringgal et al. 2020). The rise in health awareness among people leads to the elevated demand for fruits and vegetables rapidly due to their nutritional value. Fruits and vegetables comprise various nutrients, phenolic compounds, and anti-oxidants that aid in preventing many deadly diseases (Hassan et al. 2018). The main problem is connected with delivering fresh fruits and vegetables to the plate of consumers due to their perishable nature and high risk of postharvest losses in view of poor handling practices, ripening rate, and microbial contamination. Consumers demand fresh produce and judge that on the basis of visual attributes (Maringgal et al. 2020). The preservation of fruits and vegetables is a worldwide challenge; therefore, edible coatings made from natural sources are influential techniques used to overcome this trouble to some extent. Edible coatings are consumer-friendly as well as eco-friendly. Herbal edible coatings such as Aloe vera (Tripathi & Dubey 2004; Misir et al. 2014) provide health benefits and act as nutraceuticals (Raghav et al. 2016) along with the preservation of fresh produce (Maringgal et al. 2020).